When a network device includes multiple ports, separate links are used to connect physical layer (PHY) devices to corresponding medium access control (MAC) devices for each port. For example, when the network device includes four ports, four PHY and MAC devices are individually connected to each other.
For lower speeds such as 10/100 Mb/s, a medium independent interface (MII) may be used to connect each MAC/PHY pair. Gigabit Ethernet switches use a Gigabit media independent interface (GMII) to connect MAC/PHY pairs. GMII is a parallel interface that includes traces that run simultaneously at a fixed frequency between the paired MAC and PHY devices. The GMII interface works well for Gigabit Ethernet switches with one port or with relatively few ports. When additional ports are added, problems may arise relating to the relatively high number of pins, synchronization, cost and interference.
A reduced GMII (RGMII) decreased the number of pins by increasing the data frequency. The lower number of pins reduced the cost. However, running more energy through each trace increased the likelihood of interference. A serial Gigabit interface was developed using a pair of serializer/deserializers to solve problems associated with the GMII and RGMII parallel interfaces. While parallel connections allow high data rates over short distances, serial links permit longer connections and reduce synchronization issues. Despite having a higher transmit frequency, the serial interference is not as problematic because the signals do not travel in synch. The serial Gigabit interface format also allows SERDES components to be integrated on the same chip.
Referring now to FIG. 1, a network device 10 includes a medium access control (MAC) device 12 with a Gigabit MAC 14 and a physical coding sublayer (PCS) device 16. An output of the MAC device 12 is input to a first SERDES 20, which provides a serial link at a fixed data rate. A second SERDES 22 communicates with the first SERDES 20 and is connected to a PCS 26 of a PHY device 28 that also includes a PHY 30. The MAC device 12 communicates with higher level layers. The PHY 30 communicates with a medium 34. The PCS 16 may perform 8/10 bit encoding as specified by 802.3z, which increases the data rate to 1.25 Gb/s, or other suitable PCS coding may be used. A serial management interface 36 provides control information between the MAC and the PHY. Because the first and second SERDES 20 and 22 must operate at 1.25 Gb/s, a solution was required for MACs 14 that operate at lower data rates such as 10 or 100 Mb/s.